Field of the Invention
The present invention relates to an alignment sensor and to a lithographic apparatus comprising such alignment sensor.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography, when manufacturing semiconductor structures, many layers are provided on top of each other. In order to reduce overlay errors, the layers are to be aligned in respect of each other accurately, so as to be able to achieve small line widths.
Thereto, use is made of an alignment sensor. Alignment marks are provided onto the substrate in one or more of the processed layers. Each time the substrate is loaded into the lithographic apparatus, the alignment marks are measured and a position of the substrate is derived therefrom. Thereto, the alignment sensor projects optical pulses onto the alignment mark in order to be scattered by that mark. An intensity of the scattered field is after optical processing, e.g., after optical processing via an interferometer, measured by a detector, and position information (expressing the position of the mark in respect of the detector) is derived therefrom. An alignment mark may comprise, for example, a diffraction grating or a non-diffracting structure. Light that is scattered by the diffraction grating is a scattered field typically referred to as diffracted light.
As substrate structures tend to get increasingly complex, a total number of layers to be provided on the substrate tends to increase. As a result, the mark that is to be used for the alignment, may be covered with several layers provided on top of it. Depending on a structure of the layers, optical transmission of certain optical wavelengths may be affected. As a result, light pulses at such wavelengths may be adsorbed, reflected or otherwise affected by these layers. In order to increase a chance of reliably detecting the mark, use is made of multiple wavelength optical pulses.